Amythiamicins A, B, C and D are antibacterially active antibiotics which are known and produced by cultivation of a microorganism, MI481-42F4 strain belonging to the genus Amycolatopsis and which are in the form of a cyclic peptide and all are hardly soluble in water. Amythiamicins A, B, C and D are described in the "Journal of Antibiotics" Vol 47, No. 6, pp. 668-674 (1994); ditto Vol 47, No. 10, pp. 1145-1159 (1994); and ditto Vol. 48, No. 2, pp. 182-184 (1995). The chemical structures of these amythiamicins are disclosed in the "Journal of Antibitotics" Vol. 47, No. 10, pp. 1153-1159 (1994). The amythiamicin-producing MI481-42F4 strain has such microbiological properties as described in the "Journal of Antibiotics" Vol. 47, No. 6, pp. 668-674 (1994) and was deposited in the "Fermentation Research Institute" under a deposit number of FERM P-12739 in February, 1992 and has now been deposited in the "National Institute of Bio-Science and Human-Technology, Agency of Industrial Science and Technology" at Tsukuba-shi, Ibaraki-ken, Japan, under a deposit number of FERM BP-6023 since Jul. 14, 1997 in terms of the Budapest Treaty.
Amythiamicin A is given under a name of Antibiotic MI481-42F4-A in Japanese Patent Application First Publication Kokai Hei 5-310766. Also Japanese Patent Application First Publication Kokai Hei 6-263784 describes antibacterial substances MI481-42F4-A1, -A2 and -A3 or their salts as such antibacterially active substances which are obtained by decomposing the MI481-42F4-A substance in dilute hydrochloric acid, or others. Further, Japanese Patent Application First Publication Kokai Hei 7-215989 describes antibacterial substances MI481-42F4-B1, -B2 and -B3 as such antibacterially active substances which are obtained by decomposing the MI481-42F4-A substance, for example, by methanolysis, etc., followed, if desired, by esterifying the resulting product. The MI481-42F4-A3 substance corresponds to amythiamicin B, MI481-42F4-A1 substance corresponds to amythiamicin C, MI481-42F4-B1 substance corresponds to amythiamicin D, and MI481-42F4-B2 substance corresponds to amythiamicin E, respectively.
Amythiamicins A to E are all the cyclic peptides which are hardly soluble in water. They possess antibacterial activities and are generally named as "amythiamicins". Amythiamicins are expected to be useful as antibacterial agent for medical purposes, particularly to be an antibacterial agent effective against methicillin-resistant Staphylococcus aureus (MRSA). However, amythiamicins are difficult to be formulated in the form of antibacterial compositions which can contain them in their antibacterially effective concentrations, because they are hardly soluble in water. That is to say, amythiamicins possess excellent antibacterial activities against gram-positive bacteria, particularly highly resistant MRSA, but amythiamicins are substantially insoluble in water, and hence they are very much limited in their methods of administrations when they are to be used as antimicrobial agents.